Construction of the propulsion system of Europe’s first hybrid tugboat was announced at the International Tug and Salvage Conference on 17, May 2011 in Antwerp, Belgium, by Canada-based innovatory engine designers and builders Aspin, Kemp and Associates (AKA) in partnership with Dutch tug operators KOTUG International, whose Port of Rotterdam stationed tug RT Adriaan is due to be retrofitted with AKA’s proprietary ‘XeroPoint Hybrid Propulsion System’.

Harbour tugs like RT Adriaan perform a wide variety of tasks across the entire power spectrum, typically including periods of 'Stand-by for pilot’s orders', short transit passages, and then bottom-line ship berthing and un-berthing operations. Opportunities for continuous engine working at or near the high power levels that give optimum diesel engine efficiency come but rarely, indeed operational analysis has shown that tugs of this type operate at low engine loads most of the time. 

RT Adriaan: Photo credit – Frits Aalderink

The computer controlled system comprises main diesel engines, electric motors and batteries to offer a combination of modes of operation: direct diesel engine, diesel-electric and electric configurations, while  energy stored in lithium polymer batteries is used to meet low-end power requirements and also act as a bridge in transient periods when power is required but engines are not yet at speed.

The XeroPoint electrical system is based on a common DC bus that automatically maintains constant voltage. The use of large resistors is avoided by having the towing winches regenerate their power back to the DC bus and batteries.

XeroPoint Schematic:Courtesy of AKA

Such flexibility in propulsion and power options provides a number of efficient modes of operation for the tug that reflect the reality of its day-to-day working.

Standby or Idle: During standby or very low power operations banks of batteries permit the vessel to be operated in zero emission/silent mode, with no rotating machinery in operation.
Slow Transit: During low power operation the system operates as conventional diesel electric plant. Propulsion and service loads are met by a combination of diesel generators and electrical storage.
Mid Range: When vessel power requirements are within the efficient operating range of the main propulsion diesels, the vessel operates as a conventional, diesel only system. The shaft motor/generators support service loads using modern conversion technology to provide constant voltage regardless of shaft speed.
Full Power: During high power, the diesel and shaft motor work together to provide propulsion. The shaft motor augments diesel power while simultaneously increasing system responsiveness with its superior low-end torque characteristics.

Benefits of XeroPoint Claimed

    •    Improved fuel economy – no unnecessary idling
    •    Lower maintenance costs – minimised engine use
    •    Reduced emissions - engines run at best efficiency
    •    Reduced noise – operational readiness with engines shut down